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Did you ever go back out and try to find him?

Honestly it sounds to me like you may have misjudged your shot(s). Especially being your first bow "kill", no offense, but its easy to get caught up in the moment and think your arrows hit better...

I would say if you only had 4-5" of penetration and you did get his lungs, you most likely only hit 1 lung. If you haven't gone back out already, back out man. Give him PLENTY of time, the weather is...